Ah. I wasn't thinking of a Pentax x1.4 (was thinking of the Tamron PZ x1.4 which is a bit more expensive(.

From my own experiences with the 55-300 & TCs (admittedly not extensive, light isn't great here and I gave up on them quickly - especially after the testing I linked to above) I would still stick with the opinion I gave earlier, buy a used Sigma 150-500 and sell it when you get home (if you can bear to part with it).

For those interested in photography on safaris there is a superb 200+pp pdf that can be downloaded (sorry don't have the link so you'll have to do some Googling) from Uwe Skrzypczak called Wildlife Photography that covers everything from equipment and workflow to techniques, composition and even planning your trip.

These are interesting stats taken from the pdf in which he publishes a table (I've shortened it here) :

My comment : note that as a tourist your % could vary dramatically from those of a pro shooter.

and he comments :

The table shows that you can cover virtually all important focal lengths using an APS-C camera with a 400 mm lens and a teleconverter. The AF-S VR Zoom-NIK- KOR 200-400mm lens mounted on an APS-C camera represents the cream of the currently available super-telephoto lenses, and its closest focusing distance of about six feet makes it ideal for close-ups too. Good value alternatives are either the much slower 80-400 mm lens, or the even cheaper 70-300 mm lens used with a teleconverter.